South Eastern Centre Against Sexual Assault & Family Violence

What is intellectual disability?

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Section 1

Aims of this section

This section examines:

  • the definition of intellectual disability;
  • some possible developmental and social consequences of having an intellectual disability, with particular emphasis on sociosexual issues;
  • the incidence and prevalence of sexual assault of people with an intellectual disability.


Consider any experience you have had either in your current or previous positions where you have had contact with a person with an intellectual disability.

Discuss the experiences people have had and list "characteristics" of intellectual disability noted from these experiences.

Defining intellectual disability

Intellectual disability in relation to a person over the age of five years means a significant sub-average general intellectual functioning existing concurrently with deficits in adaptive behaviour and manifested during the developmental period (Intellectually Disabled Persons Services Act, 1986).


What would constitute a "significant sub-average IQ"?

A person's intellectual functioning can be assessed by formal intelligence tests which will give an Intelligence Quotient or IQ. The formal definition of "significant sub-average intellectual functioning" is an IQ which is two standard deviations below the mean, or average IQ. This means any IQ which is at or below 70 on a standardised intelligence test. (As the mean is 100, and the standard deviation is15 IQ points @ 2 standard deviations below the mean is 30 points below 100 points = 70). There are "levels" of intelligence which are also based on these assessment results. People who are at or below 70 are labelled as having a "borderline or mild' intellectual disability. The next level is "moderate", then "severe" and "profound". These labels are "clinical" definitions which define an IQ level and corresponding level of intellectual functioning. They are reasonably arbitrary and on their own will not provide adequate knowledge of the individual, their lifestyle, their strengths or wants.

What do we mean by "deficits in adaptive behaviour"?

This part of the definition refers to the person's everyday life skills which include personal care and communication. Compared with other people of the same age and culture, a person with an intellectual disability may not be as independent or capable of looking after their own everyday needs. Tests of adaptive behaviour may be used to measure this.

In assessing this area it is important to be aware of the possible lack of opportunities a person may have had, which could lead to deficits in adaptive behaviour. It does not necessarily mean the person will never be more independent or take care of their own needs. What this part of the assessment suggests is that without support, education and training the person cannot perform these tasks.

When is the developmental period?

The developmental period is defined as being, 'from conception to the eighteenth birthday'. To be assessed as having an intellectual disability, the two areas discussed above must manifest during this time.

It is important to note that only those people who are registered as having an intellectual disability under the IDPS Act, 1986 will have been assessed under these criteria. It is estimated that 1% of the Victorian population has an Intellectual Disability, (approximately 40,000 Victorians). More than three-quarters of these people have a mild intellectual disability. Many of these people would not be using services which are specifically for people with an intellectual disability, and many of them would not have any formal support from the Department of Human Services. Many live independent lives in the community and those with more moderate or severe disabilities also participate in the community with the support of specialist and genetic services.

Overhead 1: Intellectual disability training

What is intellectual disability?

  • In relation to a person over the age of 5 years, intellectual disability means the concurrent existence of significant sub-average general intellectual functioning and significant deficits in adaptive behaviour each of which became manifest before the
    age of 18 years. (Disability Act 2006)

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